In recent years, along with demands for large-size liquid crystal display cells, a so-called liquid-crystal dropping technique (liquid-crystal One Drop Filling; ODF), which has higher productivity, has been proposed as a manufacturing method of a liquid-crystal display cell (see Japanese Patent Application Laid-Open Nos. 63-179323 and 10-239694). In these methods, a liquid crystal display cell in which a liquid crystal is sealed is manufactured by dropping the liquid crystal inside a bank of a sealant for liquid crystals formed on one substrate, thereafter bonding the other substrate thereto.
In the liquid-crystal dropping technique, however, the sealant for liquid crystals in uncured state is made in contact with the liquid crystal, with the result that there is a problem that, upon manufacturing a liquid crystal display cell, some components of the sealant for liquid crystals are dissolved in the liquid crystal to cause reduction in the specific resistance of the liquid crystal; consequently, this technique has not spread as a mass-producing method for liquid crystal display cells.
With respect to a curing method after the bonding process of the sealant for liquid crystals in the liquid-crystal dropping technique, three methods including a thermo-curing method, a photo-curing method and a photo-thermo-curing method, have been proposed. The thermo-curing method has problems in that liquid crystal tends to leak from the sealant for liquid crystals that is being cured with reduced viscosity due to expansion of the heated liquid crystal, and in that some components of the sealant for liquid crystals with the reduced viscosity tend to be dissolved in the liquid crystal, and these problems are difficult to be resolved with the result that this technique has not been practically used.
Here, with respect to the sealant for liquid crystals to be used in the photo-curing method, two kinds of photopolymerization initiators, that is, a cation polymerizable type and a radical polymerizable type, have been proposed. With respect to the sealant for liquid crystals of the cation polymerizable type, since ions are generated upon photo-curing, the ion components are eluted in the liquid crystal in a contact state when the sealant of this type is used in the liquid-crystal dropping technique, resulting in a problem of a reduced specific resistance in the liquid crystal. Another problem with both of the photo-curing methods of the cation polymerizable type and the radical polymerizable type is that since a light-shield portion in which the sealant for liquid crystals is not exposed to light is left due to a metal wiring portion of an alley substrate of the liquid crystal display cell and a black matrix portion of a color filter substrate, the corresponding light-shield portion is uncured.
As described above, the thermo-curing and photo-curing methods have various problems, and in actual operation, the photo-thermo curing method is considered to be the most practical technique. The photo-thermo curing method is characterized by that the sealant for liquid crystals sandwiched by substrates is irradiated by light for primary curing, and thereafter heated for secondary curing. With respect to properties required for the sealant for liquid crystals to be used for the photo-thermo curing method, it is important to prevent the sealant for liquid crystals from contaminating the liquid crystal in respective processes before and after the light irradiation as well as before and after the heat-curing processes, and it is necessary to properly address the problem with the above-mentioned light-shield portion, that is, the problem of elution of the sealant components into the liquid crystal when the portion uncured by light irradiation is thermally cured. The following solutions to the problems are proposed: (1) a low-temperature fast curing process is carried out prior to the elution of the sealant components; and (2) the sealant is made from components that hardly elute into the liquid crystal compositions. Of course, the low-temperature fast curing process simultaneously causes degradation in the pot life during use, resulting in a serious problem in practical use. For this reason, in order to achieve a sealant for liquid crystals that provides longer pot life, and hardly contaminates liquid crystals, it is necessary to comprise components that are hardly eluted into the liquid crystal composition. However, commonly well known epoxy resins, such as a bisphenol A type epoxy resin and a bisphenol F type epoxy resin, have a good compatibility with liquid crystals with the result that these resins are not suitable for the constituent component for the sealant from the viewpoint of contamination-preventive property.
Japanese Patent Application Laid-Open No. 2001-133794 has proposed that a partially (meth)acrylated-bisphenol A-type epoxy resin disclosed in Japanese Patent Application Laid-Open No. 5-295087 should be used as a main resin component for the sealant for liquid crystals for use in the liquid-crystal dropping technique. In this case, however, although the (meth)acrylated resin has reduced solubility to liquid crystals, the degree of the reduction is not sufficient, and it is also difficult to solve a problem of the un-reacted remaining raw epoxy resin that contaminates liquid crystals.
As described above, the conventionally proposed photo-thermo curing type sealant for liquid crystals used in the liquid-crystal dropping technique is far from satisfying all the properties such as liquid crystal contamination-preventive property, adhesive strength, workable time at room temperature and low-temperature curing property.
The objective of the present invention is to develop a sealant for liquid crystals to be used for a liquid crystal display device to be manufactured by a liquid-crystal dropping technique, and more specifically, to develop a sealant for liquid crystals to be used for a liquid crystal display device to be manufactured by the liquid-crystal dropping technique comprising dropping a liquid crystal inside a bank of a sealant for liquid crystals formed on one substrate, thereafter bonding the other substrate thereto, irradiating a liquid-crystal sealed portion with light, and then heat-curing it. In other words, the objective of the present invention is to provide a sealant for liquid crystals which hardly contaminates liquid crystals throughout the manufacturing processes, shows excellent coatability, bondability and adhesive strength when applied to a substrate, and has long workable time (pot life) at room temperature and excellent low-temperature curing property.